In automatic transmissions with automated, wet start clutches, the general practice is to convert a typical mechanical transmission into an emergency operation, for example if following the failure of a transmission control apparatus, which alleviates the failure of power in a transmission in such a manner the start clutch, now unpowered, is caused to disengaging. Especially, in a case of driving in heavy traffic, this strategy can lead to serious results, since the vehicle has become capable of no more than a powerless, forward rolling behavior.
Engagement of the wet start clutch in such a condition of driving, because of technological safety reasons, is also impossible since the shutdown of power remains in force only because of a hydraulic clutch activation pressure. Insofar as the vehicle's drive motor stalls itself upon the diminution of the travel velocity with the engaged clutch, there is still a low residual speed at which the important motor driven co-acting aggregates, such as brake action reinforcement or power steering pumping can no longer properly function.
In a typical automatic transmission with a dry start clutch, during mechanical emergency operation, the clutch is engaged so that the output torque of the vehicle remains in a technological connection with the motor output until the vehicle comes to a standstill. In such a case, it is true that no dangerous, critical conditions of driving exist for the vehicle although, following the standstill, the vehicle can no longer be brought into motion or even pushed into a safer location.
In accordance with the foregoing, obviously there exists a need for a control apparatus for a start clutch of an automatic transmission of a motor vehicle which, in dependency upon the speed of rotation of the motor and/or the speed of rotation of the output drive of the transmission, the power flow in the drive string is first interrupted when a predetermined threshold speed of rotation of the motor and/or transmission output speed of rotation is understepped. In this way, no stalling of the motor of the vehicle occurs, the auxiliary aggregates remain operable and the driver is still given the advantage of removing himself and his vehicle from a possibly dangerous driving zone. Additionally by way of such a control system, even an induced movement of the vehicle, which is then in standstill, is possible since an otherwise movement blocking, power-flow connection between the motor and the transmission is broken.
Based on this background, DE 199 43 939 A1 discloses a hydraulic emergency control for a stepless transmission, wherein a dedicated clutch to the transmission becomes separable and engagable according to the speed of rotation of the vehicle motor. Thereby, in the case of a disturbance, a renewed stalling of the drive motor can be avoided upon the understepping of a defined threshold value of rotation speed and as well, a startup upon the overstepping of the threshold is enabled. According to the design of the emergency control, the possibility exists that the speed of rotation related signal can be engendered, for example as (1) a hydraulic pressure, as (2) a pneumatic pressure or as (3) an electrical voltage; any one of which can be functional.
Additionally, DE 102 38 104 A1 teaches a procedure for controlling an emergency shifting program for an automatic transmission with a start clutch, which is particularly designed to allow a realization of an emergency operation upon the standstill of the vehicle, as well as permitting a by-pass of the lowering of the speed of rotation of the motor to a point below the stalling threshold. In the case of this procedure, provision has been made that the emergency shifting program is controlled by a signal related to the vehicle travel speed and/or to the speed of rotation of the motor, which is computer processed by valve-logic and acts in such a manner that the motor operation is interrupted only in the compression stage in order to prevent the motor from stalling.
Accordingly, the purpose of the invention is the creation of a control valve arrangement for controlling of a start clutch of an automatic transmission where, during an emergency control operation, the start clutch can be disengaged if the speed of rotation of the motor and/or the output speed of rotation of the transmission, i.e., the travel speed of the vehicle, falls below a predetermined threshold value. The purpose encompasses the fact that this valve arrangement is to be economical in manufacturing costs, simple in design and reliable under emergency conditions.